Method and apparatus for acquiring an X-ray image using a solid state device

ABSTRACT

An X-ray image capture element includes a dielectric substrate layer having a top surface and a bottom surface. A plurality of transistors is arrayed adjacent the top surface of the dielectric layer. A plurality of charge storage capacitors is also arrayed adjacent the top surface of the dielectric layer, each capacitor having a conductive inner microplate connected to at least one of the transistors. Conductive address lines and sense lines are disposed adjacent the top surface of the dielectric layer for electronically activating the transistors and individually accessing each of the capacitors. A photoconductive layer is disposed over the transistors, address and sense lines, and a top conducting layer is disposed over the photoconductive layer opposite the dielectric layer. The image capture element also includes a plurality of charge barrier layers disposed adjacent, respectively, the top surface of each of the inner microplates, and a barrier dielectric layer disposed between and coextensive with the photoconductive layer and the top conducting layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a method and apparatus for capturingdigital radiographic images. More particularly, the present inventionrelates to a method and associated apparatus for capturing and readoutof electrical charges representing a latent radiographic image in aunique microcapacitor matrix panel to obtain an electrical signalrepresenting a radiogram.

2. Description of the Related Art

Traditional radiography employs a silver halide photosensitive film in alight tight cassette enclosure, to capture a latent radiographic image,which is subsequently rendered visible following chemical developmentand fixing. Because silver halide film is not very sensitive to X-rayradiation, and large exposures are required to obtain an image, mostapplications use a combination of an intensifying screen comprising aphosphor layer, with the silver halide film to achieve lower exposures.

Radiograms have also been produced by capturing a latent radiographicimage using a photoconductive plate in a xeroradiographic process. Inthis instance, a photoconductive plate sensitive to X-ray radiationcomprising at least a photoconductive layer coated over a conductivebacking layer is first charged by passing under a charging station whichgenerates corona ions. Positive or negative charge is uniformlydeposited over the plate surface. The plate is next exposed to X-rayradiation. Depending on the intensity of the incident radiation,electron hole pairs generated by the X-ray radiation are separated by afield incident to the charge laid over the surface and move along thefield to recombine with the surface charge. After X-ray exposure, alatent image in the form of electrical charges of varying magnituderemain on the plate surface, representing a latent electrostaticradiogram. This latent image may then be rendered visible by toning andpreferably transferring onto a receiving surface for better viewing.

More recent developments include the use of an electrostatic imagecapture element to capture a latent X-ray image, the element comprisinga photoconductive layer over a conductive support, the photoconductivelayer also covered by a dielectric layer, and the dielectric layerovercoated with a transparent electrode. A biasing voltage is appliedbetween the transparent electrode and the conductive support to chargethe element which is a large parallel plate capacitor. While the biasvoltage is applied, the element is exposed to image wise modulated X-rayradiation. Following exposure, the bias is removed and a latent image ispreserved as a charge distribution stored across the dielectric layer.The problem with this element structure is that the latent imagerepresented by local charge variations is a very small signal chargethat must be extracted in the presence of random noise in the totalcapacitive charge in the full plate. Signal to noise ratio is typicallypoor.

In an effort to improve the signal to noise ratio, the transparentelectrode is laid over the dielectric layer as a plurality of pixel sizemicroplates having an area commensurate with the area of the smallestresolvable element in the image. In this manner, the overall platecapacity is reduced and the signal extracted per picture element has abetter signal to noise ratio. Methods to readout the latent imageinclude, inter alia, scanning the length of the transparent electrodewith a laser beam while reading the charge flow from each of themicrocapacitors formed between the microplates and the conductive plate.While this element is a vast improvement over the continuous electrodestructure covering the full plate, the mode of use of this plate issomewhat complex particularly with respect to the manner in which theoriginal charging of the microplates occurs.

SUMMARY OF THE INVENTION

The present invention comprises an X-ray image capture element includinga dielectric substrate layer having a top surface and a bottom surface.A plurality of transistors is arrayed adjacent the top surface of thedielectric layer. A plurality of charge storage capacitors is alsoarrayed adjacent the top surface of the dielectric layer, each capacitorhaving a conductive inner microplate connected to at least one of thetransistors. Conductive address lines and sense lines are disposedadjacent the top surface of the dielectric layer for electronicallyactivating the transistors and individually accessing each of thecapacitors. A photoconductive layer is disposed over the transistors,address and sense lines, and a top conducting layer is disposed over thephotoconductive layer opposite the dielectric layer. The image captureelement also includes a plurality of charge barrier layers disposedadjacent, respectively, the top surface of each of the innermicroplates, and a barrier dielectric layer disposed between andcoextensive with the photoconductive layer and the top conducting layer.

The present invention is further directed to a method for using theX-ray image capture element to capture a radiogram by exposing thephoto-conductive layer to imagewise modulated radiation and determiningthe magnitude of electrical charges provided therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of an X-ray image captureelement in accordance with the present invention.

FIG. 2 is a schematic top view of the X-ray image capture element shownin FIG. 1.

FIG. 3 is a schematic cross sectional view of cassette for using anX-ray image capture panel in accordance with the present invention.

FIG. 4 is a schematic elevation view of an arrangement for using anX-ray image capture panel in accordance with the present invention forcapturing a X-ray image.

FIG. 5 represents an electrical equivalent of an element in accordancewith this invention after an initial operating bias voltage is applied,prior to exposure to X-ray radiation.

FIG. 6 represents an electrical equivalent of an element in accordancewith this invention immediately after exposure to X-ray radiation andafter the operating voltage is removed.

FIG. 7 is a block diagram of an arrangement for the capture and displayof a radiogram using the X-ray image capture panel of the presentinvention.

FIG. 8 represents an electrical equivalent of an element in accordancewith this invention just after the bias voltage is reversed and loweredto a negative potential.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an X-ray image capture apparatus, element or panel 16having a dielectric substrate layer 12 with a thickness to facilitatehandling of the panel 16. Over the dielectric substrate layer 12 is afirst plurality of discrete minute conductive electrodes 18 (ie., 18a,18b, 18c, . . . 18n) referred to herein as microplates 18n. Preferably,the microplates 18n are made of aluminum. The technology to produce suchmicroplates 18n is well known in the art. The dimensions of themicroplates 18n define the smallest picture element (pixel) resolvableby the element 16. They are deposited on the substrate dielectric layer12, typically, though not necessarily, using thermal deposition orsputtering techniques and can be made of a very thin film of metal suchas gold, silver, copper, chromium, titanium, platinum and the like. Overthis first plurality of microplates is applied a capacitive dielectricmaterial 19, preferably comprised of silicon dioxide; other materialssuch as silicon nitride may be used. Also deposited on the dielectricsubstrate layer 12 is a plurality of transistors 5 having two electrodes23 and 14 and a gate 21. Further shown in FIG. 1 is a second pluralityof microplates 4 (ie., 4a, 4b, 4c, . . . 4n) referred to herein asmicroplates 4n. They are deposited on the dielectric substrate layer 12typically, though not necessarily, using vacuum thermal deposition orsputtering techniques, and can be made of a very thin film of metal suchas gold, silver, copper, chromium, titanium, platinum and the like.Preferably, the microplates 4n are made of aluminum or indium-tin oxide.

FIG. 2 shows at least one transistor 5 connecting each microplate 4n toan Xn line 11. Each transistor 5, typically a FET transistor, has itsgate connected to an Xn line 11 and its source or drain connected to aYn line 13. A charge storage capacitor 6 is formed by the microplates 4nand 18n and capacitive dielectric material 19. Each microplate 4n isalso connected to electrode 14 of transistor 5. Each microplate 18n isconnected to electrical ground. Each transistor 5 serves as abi-directional switch allowing current flow between the Yn line 13 senselines and the charge storage capacitor 6 depending on whether a biasvoltage is applied to its gate through Xn address lines. The transistor5 preferably comprises a hydrogenated amorphous-silicon layer 15, aninsulating layer 99, a conductive gate 21 and the two conductiveelectrodes, one electrode 23 being connected to the Yn sense lines 13and the other electrode 14 to the microplates 4n as schematicallydepicted in FIG. 1. Each transistor could also use crystalline silicon,polycrystalline silicon or cadmium selenide. Each transistor 5 is alsocovered with a passivation layer 98 and can be shielded from actinicradiation using a dielectric substrate layer 12 or by using additionallayers. By actinic radiation, for purposes of describing the presentinvention, is meant ultraviolet, infrared, or visible radiation, butexcludes X-ray radiation and gamma-radiation. The technology for thecreation of the transistors 5 and charge storage capacitors 6 is wellknown in the art and not a subject of the present invention. See, forinstance, "Modular Series on Solid State Devices," Volume 5 ofIntroduction to Microelectronics Fabrication by R. C. Jaeger, Publishedby Addison-Wesley in 1988.

In the spaces between the microplates 4a, 4b, 4c . . . 4n, conductiveelectrodes or X1, X2, . . . Yn address lines 11 and conductiveelectrodes or Y1, Y2, . . . Yn sense lines 13 are laid out. The Xn lines11 and Yn lines 13 lines are shown laid out generally orthogonally toeach other in the spaces between the outer microplates 4n. Theorientation of the Xn lines 11 and Yn lines 13 is a matter of choice.The Xn address lines 11 are individually accessible through leads orconnectors not specifically illustrated in the drawings, along the sidesor edges of the panel 16.

For fabrication purposes, the Xn lines 11 and Yn lines 13 may beconstructed from the same aluminum layer used for fabricating themicroplates 4n. Since the Xn lines 11 and Yn lines 13 must notelectrically contact each other where they cross over, the Yn lines 13may be created after placing an insulating layer not shown in the figureover the Xn lines 11.

Each Yn line 13 is also connected to a charge amplifying detector 36.The detector may comprise an operational amplifier wired to measure thecharge in a capacitive circuit to which the charge from themicrocapacitors is directed, and which produces a voltage outputproportional to such charge. The output of detectors 36 may be sampledsequentially to obtain an output signal and the technology to do this isalso well known in the art.

Over the top surface of the microplates 4n there is applied a chargeblocking layer 10. The charge blocking layer 10 is preferably providedby an aluminum oxide layer formed on the surface of the microplates 4nalthough other blocking interfaces may also be used. The subsequentcoating thereon of a selenium photoconductive layer 8 produces an X-rayabsorption layer. In addition, the combination of layers 4n, 10, and 8behaves as a blocking diode, inhibiting one type of charge flow in onedirection. The charge blocking layer 10 must have sufficient thicknessto prevent charge leakage. In the preferred embodiment of the presentinvention, charge blocking layer 10 should have a thickness greater than100 Angstroms (0.01 micrometers).

Coated over the charge blocking layer 10, the transistors 5 and the gateand sense lines is a photoconductive layer 8 having a back surface incontact with the microplates 4n, and a front surface. Thephotoconductive layer 8 preferably exhibits very high dark resistivityand may comprise amorphous selenium, lead oxide, cadmium sulfide,mercuric iodide or any other such material, including organic materialssuch as photoconductive polymers preferably loaded with X-ray absorbingcompounds, which exhibit photoconductivity.

In the context of the present invention, exhibiting photoconductivitymeans that upon exposure to X-ray radiation, the photoconductivematerial exhibits reduced resistivity relative to that in the absence ofsuch exposure. The reduced resistivity is in reality the effect ofelectron hole pairs generated in the material by the incident radiation.Because the capacitive time constant of a capacitor is proportional tothe resistance of the capacitor, the capacitor formed by suchphotoconductive material has a reduced time constant upon exposure. Thisis electrically represented in FIG. 6. by placing a resistor 51 and aswitch 52 in parallel with the capacitor formed by the photoconductivematerial. Before exposure to radiation, the resistance of thephotoconductive material is effectively infinite; in schematic, then,equivalent to an open switch and the discharging resistor is noteffective. During exposure, the resistance of the photoconductivematerial is lowered, equivalent to a closed switch putting thedischarging resistor in parallel with the photoconductive capacitor.Preferably, the charges moving across the photoconductive layer aredirectly proportional to the intensity of the incident radiation.

The photoconductive layer 8 should be chosen of sufficient thickness toabsorb the incident X-ray radiation, or a substantial portion thereof,to provide high efficiency in radiation detection. The specific type ofmaterial selected will further depend upon the desired charge generationefficiency and charge transport property, and the desired simplicity ofmanufacture. Selenium is one preferred material.

A dielectric layer 17 is added on the top front surface of thephotoconductive layer 8. In the preferred embodiment of the presentinvention, dielectric layer 17 should have a thickness greater than onemicron. Mylar® (i.e., polyethylene terephthalate) film with a thicknessof 25 micrometers may be used for layer 17, although layers of otherthicknesses are suitable. A final front layer 9 of conductive materialtransparent to X-ray radiation is formed over the dielectric layer 17.

The dielectric layer 17, the photoconductive layer 8 and the chargestorage capacitors 6n form three microcapacitors in series. A firstmicrocapacitor is created between the front conducting layer 9 and thefront surface of the photoconductive layer 8, and a secondmicrocapacitor between that same photoconductive layer 8 and themicroplates 4n, and the third capacitor being the charge storagecapacitor 6n formed between microplates 4n and 18n.

The entire element 16 can be made by depositing successive layers ofconductors 18n, insulator 19, microplates 4n, blocking layer 10,photoconductor 8, insulator 17, and conductor 9 upon a dielectricsubstrate layer 12. The FETs 5 are built in the spaces between themicroplates 18n on the dielectric substrate layer 12. Fabrication may beaccomplished by plasma-enhanced chemical vapor deposition, vacuumdeposition, lamination, sputtering or any other known technique usefulto deposit even-thickness films.

In practice, a panel 16 may be fabricated beginning with a commerciallyavailable thin film transistor panel which comprises a dielectricsubstrate layer 12, transistors 5, and Xn lines 11 and Yn lines 13.Commercially available panels used in making liquid crystal displays area convenient starting point for building the panel 16 in accordance withthe present invention. Charge storage capacitors 6 are formed over theouter microplates 18n and between the Xn lines 11 and Yn lines 13. Thephotoconductive layer 8 is coated over the charge blocking layer 10. Thedielectric layer 17 and top conductive layer 9 are formed on thephotoconductive layer 8 to complete the panel 16.

In a preferred embodiment, the conductive top layer 9, the dielectriclayer 17, and the photoconductive layer 8 are continuous layers.However, it is within the contemplation of the present invention for oneor more of the layers overlying the microplates 18n to comprise aplurality of discrete portions, formed in registration, for instance, byetching.

In FIG. 2, the Xn lines 11 terminate to a switching means comprising afirst plurality of switches 32 that allow switching the Xn lines 11 to afirst position A, and a second position B. Preferably, the switchingmeans comprise electronically addressable solid state switches which maybe either external or integral with the element 16. A bias voltage isapplied over line 33 to all Xn lines 11 simultaneously when the Xn lines11 are in the first position A. The bias voltage on the Xn lines 11 isapplied to the gates of all the transistors 5 to change the transistors5 to a conductive state to allow current to flow between source anddrain.

When the switches 32 are in the second position B, lines Xn 11 areindependently addressable over lines 35 and are no longerinterconnected. Means to effectuate such sequential switching are notshown. Such means are well known in the art and not of particularimportance to this invention as any convenient switching arrangement maybe selected without altering the scope of this invention. Switches 32may be controlled by line 37.

Charge detectors 36 may comprise an operational amplifier wired tomeasure the charge in a capacitive circuit in which the charge from themicrocapacitors produces a voltage output proportional to such charge.The output of detectors 36 may be sampled sequentially to obtain anoutput signal and the technology to do this is also well known in theart.

In FIG. 1, in addition to the circuitry discussed above connected to thepanel 16 and Xn lines 11 and Yn 13 lines addressing means discussedabove, there is an additional connection provided for accessing thefront conductive layer 9 and the first plurality of microplates 18n inorder to electrically connect the front conducting layer 9 and the firstplurality of microplates 18n to a power supply 27 capable of providing aprogrammable series of variable voltages.

FIG. 3 shows an arrangement in which a cassette or enclosure 22 is usedto shield the image capture element 16 from exposure to actinicradiation, much in the manner a cassette shields an X-ray film. Thecassette 22 is made of material which is transparent to X-rays. Toobtain a latent radiographic image, the element 16 is placed in thecassette 22. The cassette 22 is placed in the path of informationmodulated X-ray radiation in a manner similar to the way a traditionalcassette-photosensitive film combination is positioned. Means 34 areincluded to allow electrical access to switch contacts for switch 32,and their respective control lines 33, 35 and 37 as well as power supply27.

FIG. 4 shows an schematic arrangement in which a source of X-rayradiation 44 provides a beam of X-rays. A target 48, i.e., a patient inthe case of medical diagnostic imaging, is placed in the X-ray beampath. The emerging radiation through the patient 48 is intensifymodulated because of the different degree of X-ray absorption in thetarget 48. The modulated X-ray radiation beam 46 is intercepted by thecassette 22 containing element 16. X-rays which penetrate the enclosure22 are absorbed by the photoconductive layer 8.

In operation, the switches 32 are first placed in position A where abias voltage, typically 5 volts, is simultaneously applied to all Xnlines 11. In addition, a voltage of typically 5 volts is applied to anarray reset line 91 causing all array reset transistors 93 to becomeconductive. All charge storage capacitors 6 are electrically shorted toground through the array reset transistors. Also, all charge amplifiers36 are reset through line 39. An initial operating DC voltage such as1000 v is applied at a controlled rate to the top conducting layer 9.

FIG. 5 is an simplified equivalent electric circuit of the dielectriclayer 17, the photoconductor layer 8 and the charge storage capacitor 6forming three microcapacitors in series before application of theimpinging radiation. In parallel with the photoconductor 8, there isshown a switch 52 and a resistor 51 representing the effect of theelectron hole pair generation and transport in the photoconductive layer8 on the capacitance of that capacitor to be described next. When aninitial positive operating voltage is connected across the element 16 asshown in FIG. 5, in the absence of X-ray radiation, and with transistors5 and array reset transistors 93 turned to a conductive state, theequivalent of closing the swotch 53, no charge will be accumulated inthe charge storage capacitors 6. In the described structure, this willresult in two different voltages appearing across the capacitors, oneacross the microcapacitors representing the photoconductor layer 8, andthe second across the microcapacitors representing the dielectric layer17. If, for instance, the applied voltage source 27 is 1000 volts, itcould be distributed across the two capacitors as 100 volts across thedielectric 17, and 900 volts across the photoconductor 8. Uponstabilization of the electric field, the voltage on the Xn lines biasingthe transistors 5 is changed to a second operating voltage causing thetransistors 5 to become non-conductive, by placing switches 32 inposition B. The array reset transistors 93 are also caused to becomenon-conductive by a similar process. This is equivalent to opening theswitch 53.

FIG. 6 shows the effect on the voltage redistribution pattern ofdifferent amounts of incident radiation at different pixels. DuringX-ray exposure, image wise modulated X-ray radiation impinges on thepanel 16. The X-rays generate excess electron hole pairs within thephotoconductive layer and, in the presence of the electric field causedby the difference in voltage between the front conducting layer 9 andthe microplates 18n, holes migrate toward the interface between thephotoconductive layer 8 and the charge blocking layer 10 in the regionabove the microplates 4n. The amount of electron hole pairs generatedthroughout the photoconductive layer 8 is dependent on the intensity ofimagewise modulated X-ray radiation impinging on the image captureelement 16. Positive charges accumulate across the microstoragecapacitors 6 and change the voltage pattern, for instance, to thosevoltages depicted in FIG. 6.

In the present invention, the plurality of charge barrier layers 10 andthe barrier dielectric layer 17 are important features which preventcharge build-up on the charge storage capacitors 6 due to leakagecurrent during X-ray exposure. When the positive operating voltage isapplied to the top conducting layer 9, the barrier dielectric layer 17prevents holes from being injected into the photoconductive layer 8 fromthe conducting layer 9, and the charge barrier layers 10 preventelectrons from being injected into the photoconductive layer 8 from theinner microplates 4n, thereby preventing any resulting leakage currentacross the photoconductive layer 8 from causing additional chargebuild-up on the storage capacitors 6 which is not due to the X-rayimage. Consequently, the resulting X-ray image is not affected by chargebuild-up due to leakage current, and the resolution of the X-ray imageis enhanced.

After a predetermined time period the X-ray flux is interrupted andX-rays no longer impinge on the element 16. The application of theinitial operating voltage to the top conducting layer 9 is then removed,thus capturing a radiographic image in the element 16 in the form ofstored charges in microcapacitors formed by the microplates 4n and thedielectric 19 and the microplates 18n.

Following removal of the initial operating voltage from the element 16,the cassette 22 may be handled in the presence of actinic radiationwithout loss of the stored image information contained in it as a chargedistribution in the microcapacitors across the dielectric blocking layer19 since the transistors 5 are shielded from actinic radiation andmicroplates 4n are thus isolated from each other.

Referring again to FIG. 2, each of the Xn lines 11 is sequentiallyaddressed by applying an appropriate bias voltage to the line and thusto the gate of the FETs 5 connected to the addressed Xn line 11. Thisrenders the FETs 5 conductive and the charges stored in thecorresponding charge storage capacitors 6 flow to the Yn 13 lines and tothe input of charge detectors 36. Charge detectors 36 produce a voltageoutput proportional to the charge detected on the line Yn 13. The outputof the amplifying charge detectors 36 is sequentially sampled to obtainan electrical signal representing the charge distribution in themicrocapacitors along the addressed Xn line 11, each microcapacitorrepresenting one image pixel. After the signals from one line of pixelsalong an Xn line 11 are read out, the charge amplifiers are resetthrough reset line 39. A next Xn line 11 is addressed and the processrepeated until all the charge storage capacitors have been sampled andthe full image has been read out. The electrical signal output may bestored or displayed or both.

FIG. 7 shows the signal obtained from the charge amplifier 36 preferablyconverted to a digital signal in an analog to digital (A/D) converter110. The signal is directed over line 140 from the A/D converter 110 toa computer 142. Computer 142, inter alia, directs the signal toappropriate storage means which may be both a internal RAM memory or along term archive memory 144 or both. In the process, the datarepresenting the radiogram may undergo image processing, such asfiltering, contrast enhancement and the like, and may be displaced on aCRT 146 for immediate viewing or used in a printer 148 to produce a hardcopy 150.

FIG. 8 shows how the panel 16 is prepared to capture additional X-rayimages. After a signal has been recovered, for example using the processdescribed, residual charges are eliminated by interconnecting all Xnlines 11 and again applying a bias voltage to the Xn lines 11 to renderthe transistors 5 conductive and as a result discharging completely allcharge storage capacitors. All charge amplifiers 36 are reset throughreset line 39. The initial operating voltage is reapplied to the frontconducting panel 9, and at a controlled rate the operating voltage isreduced during a predetermined time period from the operating biasvoltage to zero voltage and to a further reversed voltage which can beequal to or less than the magnitude of the original positive operatingbias voltage. This reversed voltage polarity allows holes to be injectedfrom the microplates 4n through the charge barrier layer 10 into thephotoconductive layer 8. This movement of holes through thephotoconductive layer 8 continues until the electrons previously trappedwithin the photoconductive layer 8 are recombined with holes,eliminating the previously retained imagewise modulated chargedistribution pattern. The magnitude of the reversed polarity operatingvoltage is lowered over a second predetermined time period back to zerovoltage. This erasing process is repeated until all the trapped chargesare removed and the image capture panel prepared for subsequent imagecapture operations.

What is claimed is:
 1. In an image capture element including adielectric substrate layer having a top surface and a bottom surface, aplurality of transistors arrayed adjacent the top surface of saiddielectric layer, a plurality of charge storage capacitors also arrayedadjacent the top surface of said dielectric layer, each capacitor havinga conductive inner microplate connected to at least one of saidtransistors, said inner microplate having a top surface opposite saiddielectric layer, means disposed adjacent the top surface of saiddielectric layer for electronically activating said transistors andindividually accessing each of said capacitors, a photoconductive layerdisposed over said transistors and said means for activating andaccessing, and a top conducting layer disposed over said photoconductivelayer opposite said dielectric layer, the improvement comprising aplurality of charge barrier layers disposed adjacent, respectively, thetop surface of each of said inner microplates, and a barrier dielectriclayer disposed between and coextensive with said photoconductive layerand said top conducting layer.
 2. The element in accordance with claim1, wherein each capacitor also comprises a conductive outer microplatedisposed on the top surface of said dielectric layer, and dielectricmaterial disposed over said outer microplate, said inner microplatebeing disposed over said dielectric material opposite said outermicroplate.
 3. The element in accordance with claim 2, wherein saidinner microplates comprise aluminum and wherein said charge barrierlayers comprise aluminum oxide.
 4. The element in accordance with claim2, wherein said inner microplates comprise indium-tin oxide.
 5. Theelement in accordance with claim 2, wherein each transistor is athin-film field effect transistor (FET) having a source connected to oneof said inner microplates, and a drain and a gate both connected to saidmeans for activating.
 6. The element in accordance with claim 5, whereinsaid transistor comprises a material selected from the group consistingof amorphous silicon, polycrystalline silicon, crystalline silicon andcadmium selenide.
 7. The element in accordance with claim 5, furthercomprising a passivation layer disposed between said photoconductivelayer and each of said transistors.
 8. The element in accordance withclaim 5, wherein the means for activating and accessing comprise:aplurality of discrete conductive address lines extending along thetransistors and being connected, respectively, to the gates of adjacenttransistors, and a plurality of discrete conductive sense linesextending along the transistors in a direction across the address linesand being connected, respectively, to the drain regions of adjacenttransistors.
 9. The element in accordance with claim 8, furthercomprising means for applying a variable operating voltage to the topconducting layer relative to a ground voltage maintained at the outermicroplates.
 10. The element in accordance with claim 8, furthercomprising means for switching said address lines and said sense linesfrom a first charge state to a second readout state.
 11. The element inaccordance with claim 8, further comprising charge measuring meansconnected to said sense lines for converting electrical charge stored insaid capacitors into analog signals.
 12. The element in accordance withclaim 1, in combination with an enclosure surrounding said element toform a portable electronic cassette, said enclosure including anexternal electrical cable connected to said element for providing powerto said element and for reading electrical signals from said element.13. A method for capturing a radiogram on an image capture elementcomprising:a dielectric substrate layer having a top surface and abottom surface; a plurality of transistors arrayed adjacent the topsurface of said dielectric layer; a plurality of charge storagecapacitors also arrayed adjacent the top surface of said dielectriclayer, each capacitor having a conductive inner microplate connected toat least one of said transistors, each inner microplate having a topsurface opposite said dielectric layer, each capacitor also having aconductive outer microplate disposed on the top surface of saiddielectric layer, dielectric material disposed over said outermicroplate, said inner microplate being disposed over said dielectricmaterial opposite said outer microplate; means disposed adjacent the topsurface of said dielectric layer for electronically activating saidtransistors and individually accessing each of said capacitors, saidmeans for activating and accessing including a plurality of discreteconductive address lines extending along the transistors and beingconnected, respectively, to gates of adjacent transistors, and aplurality of discrete conductive sense lines extending along thetransistors in a direction across the address lines and being connected,respectively, to drain regions of adjacent transistors; chargeamplifying means connected, respectively, to said sense lines forconverting electrical charge in said capacitors into analog signals; aphotoconductive layer disposed over said transistors and said means foractivating and accessing; a top conducting layer disposed over saidphotoconductive layer opposite said dielectric layer; a plurality ofcharge barrier layers disposed adjacent, respectively, the top surfaceof each of said inner microplates; and a barrier dielectric layerdisposed between and coextensive with said photoconductive layer andsaid top conducting layer, the method comprising:(a) bringing alladdress lines to a first bias value, connecting said inner microplatesto electrical ground potential, and setting said charge storagecapacitors to a no-signal level; (b) applying a positive operatingvoltage to the top conducting layer while maintaining said outermicroplates at electrical ground potential; (c) removing said first biasvalue from all address lines so that said charge storage capacitors arecapable of accumulating electrical charges; (d) exposing thephotoconductive layer to imagewise modulated radiation, causingelectrical charges to be generated within the photoconductive layer at adensity proportional to the amount of radiation; (e) stopping theradiation and disconnecting the initial positive operating voltageapplied to the top conducting layer, effectively creating a distributionof electrical charges within the image capture element; (f) applying asignal sequentially through the plurality of address lines to thetransistors, so as to allow the charges collected in the capacitors toflow from the capacitors into the plurality of sense lines; and (g)activating the charge amplifying means to accumulate the charges fromeach charge storage capacitor, this cumulative value later beingdigitized and stored in memory
 1. 14. A method according to claim 13,further restoring the image capture element to its original state,comprising:(a) applying a gate signal through the address lines to thetransistors so as to allow all charges remaining in the charge storagecapacitors to flow from the capacitors into the sense lines; (b)electrically grounding the charge amplifying means connected to ensurean electrically neutral ground at each charge storage capacitor; (c)reconnecting the positive operating voltage to the top conducting layerand, at a controlled rate, decreasing the voltage to an electricallyneutral ground value and then with reversed polarity continuing todecrease the voltage to a second operating negative voltage so as toneutralize any electrical charges remaining in the photoconductivelayer; and (d) reducing the reversed operating voltage back toelectrically neutral ground voltage, effectively re-initializing theimage capture element.